Allow scoping unstable features to specific targets

[ketsuban]

Currently if I want to use build-std in my .cargo/config.toml file I have to put it in an [unstable] section which applies it for every target when I only want to build the standard library for one. Would it be possible to do something like this, so that the unstable flags are only added when building for the appropriate target?

[target.thumbv4t-none-eabi]
runner = "mGBA"
rustflags = ["-Clink-arg=-Tlinker.ld"]

[target.thumbv4t-none-eabi.unstable]
build-std = ["core", "alloc"]
build-std-features = ["compiler-builtins-weak-intrinsics"]

[bravely-beep]

This would be extremely useful for my project! Would it be possible to implement a syntax similar to the one specified in the above post, even just as a stop-gap measure?

Specifically, this feature would allow rust-analyzer to analyze my code for multiple targets simultaneously, where one of them is a target that uses build-std. Currently this isn't possible, as rust-analyzer only runs a single cargo check command, and different unstable can't be configured on a per-target basis during a single check command.